The present invention relates generally to the chemical arts, and, more particularly, to chemically strengthened glass and a method for its manufacture.
Chemically strengthened glasses have been used widely in mobile devices. Typically, a glass, such as aluminosilicate glass, is treated at high temperature in molten potassium (K) salt, such as potassium nitrate (KNO3). The sodium (Na) ions in the surface layer of the glass are replaced by potassium ions.
Because potassium ions are of larger size than sodium ions, a compressive stress is created in the surface of the glass. The central portion of the glass is under tensile stress. The compressive surface increases the hardness of the glass and prevents scratches and breakage of the glass. However, if the tensile stress in the glass is above a threshold value, a small fracture in the tensile region will trigger the fracturing in the tensile region. If the surrounding compressive layer is thin, the fracture in the tensile region will penetrate the compressive layer and shatter the whole glass.
In certain applications, finer fracturing or finer shattering particles are needed, which requires a higher tensile stress in the center of the glass. However, a higher tensile stress in the glass requires a thicker compressive layer for a constant glass thickness. This thicker compressive layer will prevent the fracturing in the tensile region from penetrating through and shattering the whole glass.
However, while a thicker compressive layer will advantageously increase the resistance of glass to scratches or surface damages, it will also undesirably increase the thickness and weight of glass. Therefore, there is a long-felt but unmet need for a technique for glass which provides greater compressive strength without increasing the compressive layer thickness.